This invention pertains generally to cold cathode field emission and particularly to a multi-layer carbon-based field emitter device.
Field emitter materials are useful whenever a source of electrons is needed, in particular, for applications such as vacuum microelectronics, electron microscopy and flat panel displays. Flat panel displays that use field emission (cold cathode emission) have several potential advantages over other types of flat panel displays including; low power consumption, high intensity or brightness, large viewing angle, low projected cost, and robustness. For these reasons, field emission displays have the potential to be a low cost, high performance alternative to cathode ray and liquid crystal display technologies. One of the key issues in producing commercially viable field emitters is the development of reliable and efficient field emitter (cold cathode) materials for these devices. At the present time, field emitter materials typically require either complicated fabrication steps or high control voltages to promote emission or both. Furthermore, currently available field emitter materials have several limitations which restrict their usefulness including the lack of uniformity of emission current over the surface of the field emitter material and dynamic changes in emission with time (twinkling). It is believed that the reasons for these limitations include non-uniform current conduction through the field emitter material and the build-up of local fields due to charge separation resulting from steady-state (DC) emission.
In resistive materials at high fields current conduction can occur along filamentary conduction paths and this can lead to emission nonuniformity (e.g., the creation of discrete emission sites). During steady-state emission a space charge region can build up around these filamentary paths leading to an opposing electric field being generated. When this occurs, a greater applied field is required to maintain electron emission or the emission site will cease to emit electrons. On the other hand, a neighboring emission site in the field emitter material which was formerly inactive may "turn on" once its neighbor is "turned off". It is this progressive "turning off" and "turning on" of electron emission sites that leads to "twinkling". Thus, as more and more emission sites are "turned off" due to the build up of space charge layers, a higher voltage is required to promote electron emission.
It is known in the art to use various homogeneous materials or films for cold cathode emission applications. Included are such materials as crystalline diamond; amorphous carbon films or silicon; and patterned bulk materials, such as silicon or molybdenum "Spindt" tips. Also included are surface adsorbed or deposited layers, such as cesium or gold layers deposited on a material such as diamond or carbon to improve electron emission properties, or surface etching such as ion beam etching of diamond. However, these prior art materials or processes are either expensive to produce over the large areas necessary for field emission applications (patterned bulk material) or display undesirable properties such as high turn-on voltage, or non-uniform spatial or temporal emission characteristics, as set forth hereinabove.
One promising class of field emitter materials is amorphous carbon films containing at least some fraction of tetrahedrally-coordinated (4-fold coordinated) carbon atoms, hereinafter referred to as amorphous-tetrahedral coordinated carbon (or a-tC carbon). Such films have been shown to be excellent field emitters requiring only low turn-on voltages. However, these a-tC films can exhibit many of the aforementioned undesirable properties of other field emitter materials (e.g., localized emission sites, twinkling, etc.).
What is needed is a field emitter device that is inexpensive, easy to produce, has a low turn-on voltage and is stable in time and wherein electron emission is uniform across the field emitter device and the density of electron emission sites is increased.
Responsive to these needs, the present invention provides a field emitter device having an improved uniformity of electron emission, a high density of electron emission sites, a low turn-on voltage, is inexpensive to produce, does not require photolithographic patterning processes, and can be readily formed over large areas and a method for creating these materials.